1. Field of the Invention
This invention relates to an easily openable closure for a seamed can, and more specifically, to an easily openable closure to which an opening tab for shearing a score formed in the closure is bonded and remains fixed until the completion of the opening operation even after retorting.
2. Description of the Prior Art
An easily openable closure having an opening portion defined by a score has been widely used in cans for holding drinks and beverages such as coca cola, beer and juices. A rivet is formed in the opening portion and a pulling tab is fixed to it by riveting. Pulling of the tab results in breaking of the score and consequent detachment of the opening portion.
In the formation of such an easily openable closure, the coating on the inside surface of the closure is liable to undergo damage by severe workings such as riveting. Hence, with such a closure, no entirely satisfactory results have been obtained with regard to corrosion resistance in applications where perishable foods such as goods are filled in cans and thereafter the cans are subjected to severe treatments such as heat sterilization (retorting). This tendency is especially pronounced in easily openable closures made of aluminum. Furthermore, with contents of a high salt concentration, pitting corrosion may occur, and leakage of the contents from the pits or bacterial contamination therefrom will result.
Japanese Utility Model Publication No. 524/1976 proposes an easily openable can closure having an opening portion defined by a score and an opening tab bonded thereto.
When the gripping portion of the opening tab is lifted in this type of easily openable closure, its bonded part acts as a fulcrum and the end of the tab is stuffed into the score. As a result, shearing of the score is started. Then, by pulling the tab, the score is entirely sheared. At this time, the force required to push and tear the score acts as a moment on the bonded part whereby the bonded part will easily be broken.
In the field of canning, a filled can to which the easily openable closure is seamed is retorted at high temperatures and pressures, and the bonded part is liable to undergo degradation by attack of high temperatures, hot water or hot steam. This makes the bonded part more liable to be broken. Such a degradation tendency of the bonded part occurs likewise when a repair paint is coated on the score portion in order to repair crackings of the coated film, and then baked.
The manner of opening a can by detaching the easily openable closure differs from individual to individual. For example, when the tab is pulled without lifting it to a position nearly at right angles to the closure, a relatively large moment is exerted on the bonded part between the tab and the closure and the bonded part is liable to break easily. Furthermore, when the tab is pulled incorrectly in the tab direction, a moment also acts on the bonded part, and the bonded part becomes liable to break.